The present invention relates to a method and apparatus for measuring distances of an examined surface from a reference plane. The invention is particularly useful in applications involving measuring such distances while examining a mask pattern on a semiconductor substrate, and is therefore described below with respect to such an application, but it will be appreciated that the invention could advantageously be used in many other applications as well.
Scanning electron microscopes are commonly used for the accurate indication of critical dimensions of patterns on silicon wafers for the production of integrated circuits. These indications are done when the patterns are at the focal plane of the microscope objective. Any deviation of the pattern from the focal plane affects the indications and therefore must be known so that the focal lens of the microscope objective or the pattern position can be properly readjusted.
A number of optical systems are known for measuring distances of surfaces from a reference plane. Examples of known techniques are described in an article published by H. D. Wolpert, Photonics Spectra, June 1987, pp 165-168 and in U.S. Pat. No. 3,264,935. Such systems are generally based on focussing a beam of radiation (e.g., a laser beam) on the examined surface and reflecting the beam to a position-sensitive detector such that any deviation in the examined surface from the reference plane produces a non-symmetry in the light spot projected on the detector with respect to the reference point on the detector. The amount of the non-symmetry is measured and indicates the corresponding distance of the examined surface from the reference plane.
However, such a method is sensitive to differences in reflection of the substrate. This greatly limits its use when employed for imaging a mask pattern on a semiconductor substrate for the manufacture of integrated circuits since patterned surfaces produce significant and non-uniform scattering and diffraction, and non-uniform reflection. This results in an error, known as a drawback, in the position indication. The drawback is not constant and depends on the random geometrical and optical structure of the pattern on the examined reflecting surface.
U.S. Pat. No. 4,356,392 and U.S. patent application Ser. No. 07/909,340 filed Jul. 6, 1992 disclose systems for reducing this drawback.
According to the system described in U.S. Pat. No. 4,356,392, an auxiliary beam is focussed along a first path to a spot on the examined surface and is reflected back along a second path to a second spot at a symmetric location to the location of the first spot on the examined surface. The second path includes a retro-reflector which inverts the beam direction back along the second path to a second spot which is symmetric in its shape and its location to the first spot on the examined surface. When the examined surface is in the position of the reference surface, the first and the second spot are located at the same position on the examined surface, such that after the double reflection to the examined surface, the two halves of the beam tend to cancel out local differences in reflection.
According to the system described in U.S. patent application Ser. No. 07/909,340 filed Jul. 6, 1992, two identical beams are focussed along first and second paths to a first and a second spot respectively located at the same position on the examined surface when that surface is located in the reference plane. The first spot is imaged on a common detector via a path which includes the second path, and the second spot is imaged on the common detector via a path which includes the first path. The two images are symmetric to each other and are superimposed on each other, such that when the superimposed image is symmetric with respect to the reference point on the detector corresponding to the location of the reference plane, the superimposed images tend to cancel out local differences in reflection.
However, the systems described in U.S. Pat. No. 4,356,392 and in U.S. patent application Ser. No. 07/909,340 suffer from the following disadvantages: (1) since the drawback effect is eliminated only when the examined surface is in the location of the reference plane, the systems can be used only for detecting deviations of the examined surface from a reference plane without the ability to measure the amount of the deviations, i.e., the actual distance between the examined surface and the reference plane; and (2) the systems operate well to reduce the drawback effect only when the systems are perfectly aligned to project the two focussed beams to exactly the same spot on the examined surface. The latter requirement makes those systems very sensitive to instability of the optical components in the systems.
It would therefore be highly desirable to provide a method and apparatus for measuring the distance of examining surface from a reference plane, in which the above-described drawbacks are reduced or eliminated and in which the sensitivity of the method and the apparatus to the instability of their optical components is reduced. It would be particularly desirable to provide such a method and apparatus which could be used in examining patterned substrates for the manufacture of integrated circuits.